Stirling refrigerator and method of controlling operation of the refrigerator

ABSTRACT

In a Stirling cycle refrigerator, or in a method for controlling the operation of a Stirling cycle refrigerator, when it starts or stops being operated, or according to the detection result from position or temperature detecting means, the voltage supplied to a driving power source for driving a piston is controlled appropriately to prevent the piston from moving too far out of its movable range and thereby prevent breakage of a component resulting from collision between the piston and a displacer.

TECHNICAL FIELD

[0001] The present invention relates to a Stirling cycle refrigerator,and particularly to a free-piston-type Stirling cycle refrigerator thatdoes not employ a mechanical drive system. The present invention relatesalso to a method for controlling the operation of such a Stirling cyclerefrigerator.

BACKGROUND ART

[0002] A Stirling cycle refrigerator is a refrigerating system that isdesigned to offer the desired cooling performance by exploiting athermodynamic cycle known as the reversed Stirling cycle. In particular,free-piston-type Stirling cycle refrigerators that do not employ amechanical drive system are relatively easy to design and offerexcellent performance, and therefore their development has been quiteactive in these days with a view to putting them into practical use.

[0003]FIG. 11 is a sectional view of an example of a conventionalfree-piston-type Stirling cycle refrigerator. First, the structure ofthis Stirling cycle refrigerator will be described. Inside a cylinder 3formed substantially in the shape of a cylinder, a piston 1 and adisplacer 2, both formed in the shape of a cylinder, are arrangedcoaxially. The piston 1 is elastically supported on a pressure vessel 4by a piston support spring 5.

[0004] On the other hand, the displacer 2 has a rod 2 a formed so as toextend from a central portion thereof toward the piston 1, and this rod2 a is put through a slide hole 1 a formed so as to axially penetrate acentral portion of the piston 1. The displacer 2 is elasticallysupported on the pressure vessel 4 by a displacer support spring 6placed between the tip of the rod 2 a and the pressure vessel 4. Betweenthe rod 2 a and the slide hole 1 a, a gap is secured to permit the rod 2a to slide smoothly without friction. This gap, however, is made assmall as possible to minimize the passage of working gas.

[0005] The space formed inside the pressure vessel 4 by the cylinder 3is divided into two spaces by the piston 1. One of these spaces is aworking space 7 formed on the displacer 2 side of the piston 1, and theother is a back space 8 formed opposite to the displacer 2. The workingspace 7 is further separated into a compression space 9 and an expansionspace 10 by the piston 1 and the displacer 2. The compression andexpansion spaces 9 and 10 are connected together by a passage 12 so asto communicate with each other. In this passage 12 is arranged aregenerator 11 filled with a filling (matrix) such as metal mesh. Apredetermined amount of working gas is sealed in the pressure vessel 4.

[0006] To that side of the piston 1 opposite to the displacer 2 iscoupled a sleeve 14 made of a non-magnetic material and formed so as tohave an L-shaped section, and to the other end of the sleeve 14 isfitted an annular permanent magnet 15 along the direction in which thepiston 1 slides. Thus, inside a gap 19 between an outer yoke 17enclosing a driving coil 16 and formed so as to have a C-shaped sectionand an inner yoke 18 fitted around the outer surface of the cylinder 3,the annular permanent magnet 15 slides along the axis of the cylinder 3in synchronism with the reciprocating movement of the piston 1.

[0007] To the driving coil 16, a first lead 20 and a second lead 21 areconnected. These leads 20 and 21 are connected, through the wall of thepressure vessel 4 and via a first and a second electric contact 22 and23, to a PWM output portion 24. The annular permanent magnet 15, thedriving coil 16, the leads 20 and 21, and the yokes 17 and 18 togetherconstitute a linear motor 13. The PWM output portion 24 feeds the linearmotor 13 with an alternating current in the form of a pulse voltage.

[0008] How the conventional refrigerator structured as described aboveoperates will be described. When the PWM output portion 24 supplies analternating current via the electric contacts 22 and 23 and by way ofthe leads 20 and 21 to the driving coil 16, the driving coil 16 producesa magnetic field of which the polarities at both ends change at thefrequency of the alternating current. In the gap 19, this magnetic fieldwith changing polarities interacts with the annular permanent magnet 15,and causes attracting and repelling forces to act on the annularpermanent magnet 15 along the axis of the cylinder 3. As a result, thepiston 1, to which the annular permanent magnet 15 is fitted, movesaxially inside the cylinder 3.

[0009] Suppose that the driving coil 16 is fed with an alternatingcurrent having a sinusoidal waveform. Then, the piston 1 reciprocates bysliding along the inner wall of the cylinder 3. As a result, the workinggas in the compression space 9 is compressed, passes through theregenerator 11, where the heat of the working gas is collected, andmoves to the expansion space 10. The working gas that has flowed intothe expansion space 10 presses the displacer 2 and is expanded.

[0010] As the displacer 2 is pushed back by the resilient force of thedisplacer support spring 6, the working gas is pressed out in theopposite direction, passes through the regenerator 11, where the workinggas receives the heat collected by the regenerator 11 a half cycle ago,and returns to the compression space 9.

[0011] In this way, the reversed Stirling cycle is formed, in which thevariation in the pressure of the working medium compressed and expandedin the working space 7 causes the piston 1 and the displacer 2 toresonate with a phase difference of, typically, 90° relative to eachother according to the spring constants of the piston support spring 5and the displacer support spring 6, respectively.

[0012] However, during the operation of the refrigerator, if thepressure of the working gas varies abnormally, or the proper gas balanceis lost, the piston 1 may move beyond the tolerated amplitude asdesigned, i.e. out of its permitted range of movement. In the worstcase, the piston 1 may collide with the displacer 2 reciprocating withthe aforementioned phase difference relative thereto, leading tobreakage of a component.

[0013] Therefore, in the operation of a free-piston-type Stirling cyclerefrigerator, the alternating current that is fed to the linear motor 13needs to be controlled carefully so that the piston 1 does not movebeyond the tolerated amplitude.

[0014]FIG. 12 is a side sectional view of another example of aconventional free-piston-type Stirling cycle refrigerator.

[0015] The Stirling cycle refrigerator 115 has a piston 161 and adisplacer 162 linearly reciprocating inside a cylinder 163. The piston161 and the displacer 162 are arranged coaxially. The displacer 162 hasa rod 162 a formed so as to extend therefrom and penetrate through aslide hole 161 a formed in a central portion of the piston 161. Thepiston 161 and the displacer 162 can slide smoothly along an inner slidesurface 163 a of the cylinder 163. The piston 161 and the displacer 162are elastically supported on a pressure vessel 164 by a piston supportspring 165 and a displacer support spring 166, respectively.

[0016] The space formed by the cylinder 163 is divided into two spacesby the piston 161. One of these spaces is a working space 167 located onthe displacer 162 side of the piston 161, and the other is a back space168 located on that side of the piston 161 opposite to the displacer162. Working gas such as pressurized helium gas is sealed in thesespaces. The piston 161 is made to reciprocate with a predeterminedperiod by an unillustrated piston driver such as a linear motor. Thus,the working gas inside the working space 167 is compressed and expanded.

[0017] The variation in the pressure of the working gas compressed andexpanded in the working space 167 causes the displacer 162 toreciprocate linearly. The piston 161 and the displacer 162 are designedto reciprocate with a predetermined phase difference and with anidentical period. Here, the phase difference is determined by the massof the displacer 162, the spring constant of the displacer supportspring 166, and the operation frequency of the piston 161, if the otheroperation conditions are assumed to be the same.

[0018] The working space 167 is further divided into two spaces by thedisplacer 162. One of these spaces is a compression space 167 a locatedbetween the piston 161 and the displacer 162, and the other is anexpansion space 167 b located at the closed end of the cylinder 163.These two spaces are coupled together through a heat rejector 170, aregenerator 169, and a chiller 171. The working gas in the expansionspace 167 b produces cold at a cold head 172 located at the closed endof the cylinder 163. The principles of the working of the reversedStirling refrigerating cycle, such as how it produces cold, is wellknown, and therefore their explanations will be omitted.

[0019] Here, gas bearings are used as bearing mechanisms between thepiston slide surface 161 b and the cylinder slide surface 163 a andbetween the displacer slide surface 162 a and the cylinder slide surface163 a. The bearing effect of these gas bearings results from the workinggas compressed by the reciprocating movement of the piston 161 fillingthe gap between the piston 161, the displacer 162, and the cylinder 163and thereby permitting their slide surfaces slide without making contactwith each other.

[0020] Japanese Patent Application Laid-Open No. H7-180919 discloses amethod of starting the operation of a crank-type Stirling cyclerefrigerator. According to this method, the frequency and the voltageare controlled linearly from the very start of the operation of theStirling cycle refrigerator so as to prevent excessive current at thestart of operation.

[0021] However, with a free-piston-type Stirling cycle refrigerator 115as shown in FIG. 12, in which the spring constant of the displacersupport spring 166 and the masses of the displacer 162 and the displacersupport spring 166 are so set as to produce resonance at the optimallytuned frequency at which the maximum cooling performance is obtained,starting its operation at previously set fixed frequency and voltagefrom the start results in greatly missing the resonance point. Thiscauses abnormal oscillation and thus breakage of the Stirling cyclerefrigerator 115.

[0022] Moreover, for example, when a refrigerator-freezer apparatusincorporating the free-piston-type Stirling cycle refrigerator 115 hasjust been installed, and thus the temperature inside the apparatus isclose to normal temperature, starting the operation of the refrigeratorputs a heavy load on it. Thus, if an excessive input is fed to therefrigerator to make it operate at high power immediately after itstarts operating, since the pressure of the working gas has not yet comeinto a steady state (in which the heat rejector 170 and the chiller 171of the Stirling cycle refrigerator 115 have a predetermined temperaturedifference), there is a risk of the piston 161 and the displacer 162interfering and colliding with each other.

[0023] When the operation of the free-piston-type Stirling cyclerefrigerator 115 is stopped, if the supply of electric power thereto isshut down suddenly, the Stirling cycle refrigerator 115 stops operatingsuddenly. This causes a large variation in the pressure of the workinggas, and therefore there is a risk of the piston 161 and the displacer162 interfering and colliding with each other.

[0024] When the cooling performance of the free-piston-type Stirlingcycle refrigerator 115 is adjusted, typically the voltage applied to thepiston 161 is varied. The maximum amplitude of the piston 161 depends onthe structure of the refrigerator, and the voltage applied to the piston161 is controlled by a microcomputer so that the piston 161 does notmove beyond the maximum amplitude. However, if the input voltage varies,a voltage higher than the rated maximum voltage may be applied to thepiston 161. This causes the piston 161 to move beyond the designedamplitude, and therefore there is a risk of the piston 161 and thedisplacer 162 interfering and colliding with each other.

[0025] Moreover, in the free-piston-type Stirling cycle refrigerator 115employing gas bearings, the gas bearing effect is not obtained inlow-speed or small-amplitude operation. This causes friction between thepiston 161 and the cylinder 163 and between the displacer 162 and thecylinder 163 as they slide, and thus shortens the life of the Stirlingcycle refrigerator.

DISCLOSURE OF THE INVENTION

[0026] An object of the present invention is to provide afree-piston-type Stirling cycle refrigerator that prevents collisionbetween the piston and displacer thereof during the operation of thefree-piston-type Stirling cycle refrigerator. Another object of thepresent invention is to provide a method for controlling the operationof a Stirling cycle refrigerator that ensures a gas bearing effect andthat prevents breakage due to abnormal oscillation of the Stirling cyclerefrigerator or collision between the piston and displacer thereof.

[0027] To achieve the above object, according to the present invention,a Stirling cycle refrigerator provided with a piston that is arrangedinside a cylindrical cylinder and that reciprocates along the axis ofthe cylinder, a driving power source that drives the piston toreciprocate, an electric power source that supplies an input to thedriving power source, and a displacer that reciprocates inside thecylinder with a predetermined phase difference relative to the piston isfurther provided with position detecting means that is arranged outsidethe movable range within which the piston is permitted to reciprocateand control means that reduces the input supplied from the electricpower source to the driving power source when the position detectingmeans detects that the piston has moved out of the movable range.

[0028] With this structure, when the position detecting means detectsthe piston reciprocating out of its movable range, the control meansaccordingly reduces the input supplied to the driving power source ofthe piston. This prevents the piston from moving too far out of itsmovable range and thereby prevents breakage of a component resultingfrom collision between the piston and the displacer.

[0029] According to the present invention, a Stirling cycle refrigeratorprovided with a piston that is arranged inside a cylindrical cylinderand that reciprocates along the axis of the cylinder, a permanent magnetthat is fitted to the piston, a driving coil that is arranged around thepermanent magnet with a gap secured in between, an electric power sourcethat supplies an alternating current to the driving coil, and adisplacer that reciprocates inside the cylinder with a predeterminedphase difference relative to the piston is further provided with aposition detecting coil that is arranged on both sides or one side ofthe driving coil coaxially therewith outside the movable range withinwhich the permanent magnet is permitted to reciprocate in a mannerinterlocked with the reciprocating movement of the piston and acontroller that varies the voltage of the alternating current suppliedto the driving coil on detecting an electromotive force appearing in theposition detecting coil when the permanent magnet moves out of themovable range.

[0030] With this structure, when the permanent magnet, which moves in amanner interlocked with the reciprocating movement of the piston, movesout of its movable range, the permanent magnet passes by the positiondetecting coil, causing an electromotive force to appear therein.According to this electromotive force, the controller varies the voltageof the alternating current supplied to the driving coil of the piston.This prevents the piston from moving too far out of its movable rangeand thereby prevents breakage of a component resulting from collisionbetween the piston and the displacer.

[0031] According to the present invention, in a method for controllingthe operation of a Stirling cycle refrigerator provided with a pistonthat is arranged inside a cylindrical cylinder, a permanent magnet thatis fitted to the piston, a driving coil that is arranged around thepermanent magnet with a gap secured in between, an electric power sourcethat supplies an alternating current to the driving coil, and adisplacer that reciprocates inside the cylinder with a predeterminedphase difference relative to the piston, when the permanent magnet movesout of the movable range within which it is permitted to reciprocate ina manner interlocked with the reciprocating movement of the piston, andas a result an electromotive force appears in a position detecting coilthat is arranged on both sides or one side of the driving coil coaxiallytherewith outside the movable range of the permanent magnet, the voltageof the alternating current supplied to the driving coil is varied.

[0032] With this method, when the permanent magnet, which moves in amanner interlocked with the reciprocating movement of the piston, movesout of its movable range, the permanent magnet passes by the positiondetecting coil, causing an electromotive force to appear therein.According to this electromotive force, the voltage of the alternatingcurrent supplied to the driving coil of the piston is varied. Thisprevents the piston from moving too far out of its movable range andthereby prevents breakage of a component resulting from collisionbetween the piston and the displacer.

[0033] According to the present invention, a method for controlling theoperation of a Stirling cycle refrigerator includes providing afree-piston-type Stirling cycle refrigerator having a piston thatreciprocates inside a cylinder by use of a gas bearing and a drivingpower source that drives the piston, and operating the Stirling cyclerefrigerator by applying a voltage to the driving power source. Here,when the Stirling cycle refrigerator starts being operated, the drivingpower source starts being operated by being fed with the lowest voltagethat permits the gas bearing to function as such, and then the voltageis gradually increased up to a predetermined voltage.

[0034] In this way, when the Stirling cycle refrigerator starts beingoperated, by first applying a low voltage thereto that barely permitsthe gas bearing to function as such and then gradually increasing thevoltage up to the predetermined voltage, it is possible to ensure thegas bearing effect, to produce resonance between the piston and thedisplacer and thereby prevent abnormal oscillation of the Stirling cyclerefrigerator, and to prevent breakage resulting from collision betweenthe piston and the displacer.

[0035] According to the present invention, a method for controlling theoperation of a Stirling cycle refrigerator includes providing afree-piston-type Stirling cycle refrigerator having a piston thatreciprocates inside a cylinder by use of a gas bearing and a drivingpower source that drives the piston, and operating the Stirling cyclerefrigerator by applying a voltage to the driving power source. Here,when the Stirling cycle refrigerator stops being operated, the voltageapplied to the driving power source is gradually reduced to the lowestvoltage that permits the gas bearing to function as such, and then thevoltage is turned to zero.

[0036] In this way, when the Stirling cycle refrigerator stops beingoperated, by first gradually lowering the applied voltage to a lowvoltage that barely permits the gas bearing to function as such and thenturning it to zero, it is possible to ensure the gas bearing effect, toproduce resonance between the piston and the displacer and therebyprevent abnormal oscillation of the Stirling cycle refrigerator, and toprevent breakage resulting from collision between the piston and thedisplacer.

[0037] According to the present invention, a method for controlling theoperation of a Stirling cycle refrigerator includes providing a Stirlingcycle refrigerator having a chiller that produces cold, a heat rejectorthat produces heat, temperature detecting means fitted individually tothe chiller and the heat rejector, a piston that reciprocates inside acylinder, and a driving power source that drives the piston, andoperating the Stirling cycle refrigerator by applying a voltage to thedriving power source. Here, the temperature detecting means detects thetemperature difference between the chiller and the heat rejector of theStirling cycle refrigerator when it is not in operation, and, thegreater the temperature difference, the faster the voltage applied tothe driving power source when the Stirling cycle refrigerator startsbeing operated is increased.

[0038] In this way, by detecting the temperature difference between thechiller and the heat rejector of the Stirling cycle refrigerator when itis not in operation and increasing, faster the greater the temperaturedifference, the voltage applied to the driving power source when theStirling cycle refrigerator starts being operated, it is possible toprevent breakage resulting from collision between the piston and thedisplacer.

[0039] According to the present invention, a method for controlling theoperation of a Stirling cycle refrigerator includes providing a Stirlingcycle refrigerator having a piston that reciprocates inside a cylinderand a driving power source that drives the piston, and operating theStirling cycle refrigerator by applying a voltage to the driving powersource. Here, when the input voltage is higher than a predeterminedvoltage, a voltage lowered down to the predetermined voltage is appliedto the driving power source.

[0040] In this way, when the input voltage from the electric powersource is higher than the predetermined voltage, by applying a voltagelowered down to the predetermined voltage to the driving power source,it is possible to control the piston so that it does not move beyond itsmaximum amplitude and thereby prevent breakage resulting from collisionbetween the piston and the displacer.

BRIEF DESCRIPTION OF DRAWINGS

[0041]FIG. 1 is a sectional view of an example of a free-piston-typeStirling cycle refrigerator according to the invention.

[0042]FIG. 2 is a block diagram of the controller of thefree-piston-type Stirling cycle refrigerator according to the invention.

[0043]FIG. 3 is a flow chart of an example of the control method of thefree-piston-type Stirling cycle refrigerator according to the invention.

[0044]FIG. 4 is a diagram showing the displacement of the piston fromthe center of its reciprocating movement and the waveform of the pulsevoltage fed to the driving coil in the free-piston-type Stirling cyclerefrigerator according to the invention.

[0045]FIG. 5 is a diagram showing the displacement of the piston fromthe center of its reciprocating movement and the waveform of the pulsevoltage fed to the driving coil in the free-piston-type Stirling cyclerefrigerator according to the invention.

[0046]FIG. 6 is a block diagram of the operation controller of arefrigerating apparatus according to the invention.

[0047]FIG. 7 is a flow chart of the operation control of therefrigerating apparatus according to the invention.

[0048]FIG. 8 is a side sectional view of a Stirling cycle refrigeratorof Example 3 according to the invention.

[0049]FIG. 9 is a flow chart of the operation start mode in Example 3according to the invention.

[0050]FIG. 10 is a flow chart of the procedure performed by themicrocomputer in Example 4 according to the invention.

[0051]FIG. 11 is a sectional view of an example of a conventionalfree-piston-type Stirling cycle refrigerator.

[0052]FIG. 12 is a sectional view of another example of a conventionalfree-piston-type Stirling cycle refrigerator.

BEST MODE FOR CARRYING OUT THE INVENTION

[0053] <<First Embodiment>>

[0054] A first embodiment of the present invention will be describedbelow with reference to the drawings. FIG. 1 is a sectional view of anexample of a free-piston-type Stirling cycle refrigerator according tothe invention. FIG. 2 is a block diagram of the controller of therefrigerator. FIG. 3 is a flow chart of an example of the control methodof the refrigerator. FIGS. 4 and 5 are diagrams showing the displacementof the piston from the center of its reciprocating movement and thewaveform of the pulse voltage fed to the driving coil. In FIGS. 1 and 2,such members as are found also in the conventional free-piston-typeStirling cycle refrigerator shown in FIG. 11 and described earlier areidentified with the same reference numerals, and their detailedexplanations will be omitted.

[0055] First, the features unique to the first embodiment will bedescribed with reference to FIGS. 1 and 2. On both sides of the drivingcoil 16, outside the movable range of the annular permanent magnet 15, apair of position detecting coils 28 and 28 is provided. These positiondetecting coils 28 simply need to produce a weak electromotive forceinduced by a change in the magnetic field, and therefore, to save space,they are each formed as a coil of one to two turns.

[0056] From the position detecting coils 28 and 28, leads 30 and 30 arelaid through the pressure vessel 4, and are connected through anamplifier 31 to a controller 32. The controller 32 includes a memoryportion 33 that receives the detection signal (the inducedelectromagnetic force) from the position detecting coils 28 and storesit, a comparator portion 34 that compares the voltage stored in thememory portion 33 with a previously set voltage, and a PWM outputportion 24 that determines an adequate voltage on the basis of theresult of comparison and feeds an alternating current having thatvoltage to the linear motor 13. The PWM output portion 24 is soconfigured as to output a pulse voltage (see FIG. 4) of which theamplitude is varied stepwise among a plurality of predetermined levels.

[0057] Next, an example of the control method of the free-piston-typeStirling cycle refrigerator structured as described above will bedescribed with reference to FIGS. 1 to 5. When the refrigerator isoperating normally, one-to-one correspondence is established between thedisplacement of the piston 1 from the center of its reciprocatingmovement and the amplitude of the alternating-current voltage fed fromthe PWM output portion 24 to the linear motor 13.

[0058] However, a sporadic change in the pressure of the working gas orthe loss of the proper gas balance causes an irregular change in theundulations of the working gas. As a result, as shown in FIG. 5, thepiston 1 may move beyond the tolerated amplitude as designed, i.e. outof its permitted range of movement. In this case, the aforementionedcorrespondence breaks, and therefore, as long as the alternating currentis kept fed to the linear motor 13 at the same power, it is not possibleto restore the increased amplitude of the piston 1 to its originallevel.

[0059] Moreover, with the amplitude of the piston 1 increased, there iseven a risk of the piston 1 colliding with the displacer 2, whichreciprocates with a phase difference of about 90° relative thereto. Thismay lead to breakage of a component. When the amplitude of the piston 1increases in this way, the annular permanent magnet 15, which moves in amanner interlocked with the reciprocating movement of the piston 1,passes inside the position detecting coils 28, and thus causes aninduced electromotive force to appear in the position detecting coils28.

[0060] Now, how the refrigerator is controlled in this case will bedescribed in more detail with reference to the flow chart of FIG. 3. Instep S1, a pulse voltage (see FIG. 4) with a constant period and aconstant amplitude is fed from the PWM output portion 24 to the linearmotor 13 so as to make the piston 1 reciprocate with the desiredamplitude. At this point, in step S2, the detection of the inducedelectromotive force appearing in the position detecting coils 28(FIG. 1) is started. The electromotive force is amplified by theamplifier 31 and is then, in step S3, stored in the memory portion 33 inthe controller 32. Then, in step S4, the electromotive force as observedat the moment is compared with a predetermined reference level by thecomparator portion 34.

[0061] If, in step S4, the electromotive force appearing in the positiondetecting coils 28 (FIG. 1) is found to be higher than the referencelevel (“N” in the flow chart), then, in step S5, the amplitude of thepulse voltage fed to the linear motor 13 is set to be one step lower.Then, back in step S1, the pulse voltage, of which the amplitude is nowone step lower, is fed from the PWM output portion 24 to the linearmotor 13. In this way, it is possible to immediately reduce theamplitude of the reciprocating movement of the piston 1 within itstolerated level.

[0062] On the other hand, if, in step S4, the electromotive force isfound to be not higher than the reference level (“Y” in the flow chart),then, in step S6, whether the electromotive force is zero or not ischecked. If, in step S6, the electromotive force is found to be notzero, then, in step S7, the amplitude of the pulse voltage fed to thelinear motor 13 is kept at its current level without being changed.Then, back in step S1, the pulse voltage, of which the amplitude isunchanged, is fed from the PWM output portion 24 to the linear motor 13.In this case, although the piston 1 is reciprocating out of its movablerange, there is no risk of its colliding with the displacer 2, andtherefore there is no need to bother to change the amplitude of thepulse voltage fed to the linear motor 13.

[0063] On the other hand, if, in step S6, the induced electromotiveforce stored is found to be zero, i.e. no electromotive force is foundto have been induced, then it is assumed that the piston 1 isreciprocating within the tolerated amplitude as designed, and therefore,in step S8, the amplitude of the pulse voltage fed to the linear motor13 is set to be one step higher. Then, back in step S1, the pulsevoltage, of which the amplitude is now one step higher, is fed from thePWM output portion 24 to the linear motor 13. In this case, the piston 1is reciprocating within its movable range, but its amplitude may havelowered from the level at the start of operation for some reason.Therefore, the amplitude of the pulse voltage fed to the linear motor 13is made one step higher by way of precaution.

[0064] In the first embodiment, a pair of position detecting coils 28and 28 is arranged on both sides of the driving coil 16. The same effectis achieved, however, by arranging a position detecting coil 28 on oneside of the driving coil 16, because the amplitude increases in the samemanner on both sides as long as the center of the reciprocating movementof the piston 1 remains in a fixed position.

[0065] In the first embodiment, there is no need to use a driving powersource to drive the displacer. This helps simplify the structure of theStirling cycle refrigerator as compared with a two-cylinder-typeStirling cycle refrigerator that requires energy to make the displacerreciprocate, and also helps reduce the running costs of the refrigeratorin operation.

[0066] <<Second Embodiment>>

[0067] Next, a second embodiment of the present invention will bedescribed. Here, as a Stirling cycle refrigerator, one with a structuresimilar to that of the conventional one shown in FIG. 12 is adopted.

[0068]FIG. 6 shows a block diagram of the operation controller of arefrigerating apparatus provided with a Stirling cycle refrigerator. Avoltage supplied from an electric power source 110 is controlled throughan input voltage detecting portion 111 by a microcomputer 112, and isthen applied through a PWM (pulse width modulation) output portion 113to a Stirling cycle refrigerator 115. Information on the temperature ofthe Stirling cycle refrigerator 115 is fed from a temperature detectingportion 114 to the microcomputer 112.

[0069]FIG. 7 shows a flow chart of the operation control of therefrigerating apparatus. First, when the supply of power to therefrigerating apparatus is turned on (step S20), the microcomputer 112executes an operation start mode, whereby, according to the informationon the temperature and the like of the Stirling cycle refrigerator 115,the conditions under which to start the Stirling cycle refrigerator 115(step S21) are determined and then its operation is started (step S22).Next, when the temperature detecting portion 114 detects that thetemperature of the refrigerating apparatus has reached a predeterminedtemperature (step S23), the microcomputer 112 executes an operation stopmode, whereby, under the previously set conditions under which to stopthe Stirling cycle refrigerator 115 (step S24), the operation of theStirling cycle refrigerator 115 (step S25) is stopped. Thereafter, astime passes, when the temperature detecting portion 114 detects that thetemperature of the refrigerating apparatus has risen (step S26), themicrocomputer 112 executes the operation start mode (step S21) again torestart the operation of the Stirling cycle refrigerator 115. Now,various examples of the second embodiment will be described.

EXAMPLE 1

[0070] Example 1 is an example of implementation of the procedureperformed in the operation start mode (step S21) shown in FIG. 7 in thesecond embodiment, i.e. an example of the operation start method of theStirling cycle refrigerator 115. In the operation start mode (step S21),the piston starts being operated with a voltage previously stored as thelowest voltage that produces resonance between the piston and thedisplacer of the Stirling cycle refrigerator 115 and that permits thegas bearing to function as such, and then the voltage is increasedstepwise, for example, every second in predetermined increments until itreaches a predetermined voltage. Here, the predetermined voltage isusually a voltage determined according to the set temperature, and itsmaximum value is equal to the voltage determined by the structure of theStirling cycle refrigerator 115, i.e. the voltage that produces themaximum amplitude of the piston and the displacer.

[0071] The voltage fed to the piston at the start of operation may beany voltage higher than the lowest voltage that permits the gas bearingto function as such. However, the higher this voltage is made, thehigher the risk of the piston and the displacer interfering andcolliding with each other as result of the pressure of the working gasnot being in a steady state.

[0072] In this operation start method, the voltage may be increased inany other manner than by being increased stepwise in predeterminedincrements as time passes as described above; for example, the voltagemay be increased gradually with a predetermined gradient.

[0073] After the temperature of the refrigerating apparatus has reachedthe set temperature, the Stirling cycle refrigerator 115 may be keptoperating, without being stopped, with a somewhat lower voltage fed tothe Stirling cycle refrigerator 115 so that the refrigerating apparatusis kept at the set temperature. This helps reduce the frequency of theload put on the Stirling cycle refrigerator 115 when it starts or stopsbeing operated, and thus helps prolong its life.

[0074] With this operation start method, it is possible, in a Stirlingcycle refrigerator, to ensure the gas bearing effect, to produceresonance between the piston and the displacer and thereby preventabnormal oscillation of the Stirling cycle refrigerator, and to increasethe voltage applied thereto gradually and thereby prevent breakageresulting from collision between the piston and the displacer.

EXAMPLE 2

[0075] Example 2 is an example of implementation of the procedureperformed in the operation stop mode (step S24) shown in FIG. 7 in thesecond embodiment, i.e. an example of the operation stop method of theStirling cycle refrigerator 115. In this operation stop method, theoperation of the Stirling cycle refrigerator 115 is stopped by areversed version of the procedure performed to start its operation inExample 1. Specifically, in the operation stop mode (S24), the voltageis reduced, for example, every second in predetermined decrements untilit reaches the lowest voltage that produces resonance between the pistonand the displacer and that permits the gas bearing to function as such,and then the voltage is turned to zero.

[0076] The voltage may be turned to zero when it becomes equal to anyvoltage higher than the lowest voltage that permits the gas bearing tofunction as such. However, the higher the voltage at which therefrigerator is stopped, the greater the change in the pressure of theworking gas, and thus the higher the risk of the piston and thedisplacer interfering and colliding with each other.

[0077] In this operation stop method, the voltage may be reduced in anyother manner than by being reduced stepwise in predetermined incrementsas time passes as described above; for example, the voltage may bereduced gradually with a predetermined gradient.

[0078] With this operation stop method, it is possible, in a Stirlingcycle refrigerator, to ensure the gas bearing effect, to produceresonance between the piston and the displacer and thereby preventabnormal oscillation of the Stirling cycle refrigerator, and to reducethe voltage applied thereto gradually and thereby prevent breakageresulting from collision between the piston and the displacer.

EXAMPLE 3

[0079] Example 3 is an example of implementation of the operation startmethod of the Stirling cycle refrigerator 115, in which the optimumoperation conditions are determined separately by using differentprocedures between when the operation start mode (step S21) is executedafter information on a rise in temperature is given (step S26) in FIG. 7in the second embodiment and when the operation start mode (step S21) isexecuted immediately after the supply of power is turned on as inExample 1.

[0080]FIG. 8 shows a side sectional view of the Stirling cyclerefrigerator of Example 3, and FIG. 9 shows a flow chart of theoperation start mode in Example 3. In FIG. 8, such members as are foundalso in FIG. 12 are identified with the same reference numerals. Thechiller 171 and the heat rejector 170 are respectively fitted with, astemperature detecting means, temperature sensors 173 and 174, which areconnected to the microcomputer (not shown). The temperatures of thechiller 171 and the heat rejector 170 when the Stirling cyclerefrigerator 115 is not in operation are measured, and information onthese temperatures is fed to the operation start mode, i.e. to step S21(step S40). Then, the temperature difference between the chiller 171 andthe heat rejector 170 is calculated, and, according to the temperaturedifference, which operation start method to choose is determined (stepS41).

[0081] When the temperature difference between the heat rejector 170 andthe chiller 171 is large, for example, when only a short period haselapsed after the refrigerator stopped being operated last time, andthus the temperature of the heat rejector 170 is 30° C. and thetemperature of the chiller 171 is −20° C., it is judged that quickstarting is possible. Thus, the piston starts being operated with thelowest voltage that permits resonance between the piston and thedisplacer of the Stirling cycle refrigerator 115 and that permits thegas bearing to function as such, and then the voltage is increased atshorter intervals than in Example 1, for example every 0.25 seconds, inpredetermined increments until it reaches the predetermined voltage(step S42).

[0082] In this way, when the temperatures of the heat rejector 170 andthe chiller 171 are close to their temperatures in a steady state, thereis no risk of the piston and the displacer interfering and collidingwith each other as may occur when the pressure of the working gas is notin a steady state. Thus, the voltage can be increased quickly to attainthe set temperature in a short time.

[0083] On the other hand, when the temperature difference between theheat rejector 170 and the chiller 171 is small, for example, after therefrigerating apparatus has been out of operation for a long period,such as immediately after its installation or after the supply of powerthereto has been shut off, and thus the temperatures of the heatrejector 170 and the chiller 171 are both 20° C., it is judged thatnormal starting is possible, and therefore the voltage is increased inthe same manner as in Example 1 (step S43).

[0084] In this way, when the temperatures of the heat rejector 170 andthe chiller 171 are close to each other, the refrigerator starts beingoperated in the same manner as in Example 1 to prevent breakageresulting from collision between the piston and the displacer resultingfrom the pressure of the working gas not being in a steady state.

[0085] Whether the temperature difference between the heat rejector 170and the chiller 171 is large or small is checked against a predeterminedreference value, for example 40° C. Specifically, if the temperaturedifference is larger than this value, quick starting is chosen and, ifit is smaller, normal starting is chosen.

EXAMPLE 4

[0086] Example 4 is an example of implementation of the procedureperformed by the microcomputer 112 when the input voltage detectingportion 111 detects the input voltage causing the piston to move beyondits maximum amplitude in FIG. 6 in the second embodiment, i.e. anexample of the operation control method of the Stirling cyclerefrigerator 115. More specifically, in this operation control method,when the detected input voltage is higher than the rated maximumvoltage, a voltage lowered down to below the rated maximum voltage isfed to the piston.

[0087]FIG. 10 shows a flow chart of the procedure performed by themicrocomputer 112. Here, how much the input voltage is higher than therated voltage is calculated, and the voltage is lowered according to thedegree of excess. For example, whether or not the input voltage ishigher than the rated voltage by 10 V or more is checked (step S50),and, if the excess is 10 V or more, whether or not the input voltage ishigher than the rated voltage by 15 V or more is checked (S51). If theexcess is less than 15 V, the output voltage is made one step (forexample 10 V) lower (step S52). If the excess is 15 V or more, theoutput voltage is made two steps (for example 20 V) lower (step S53). Ifthe input voltage is found to be higher than the rated voltage by lessthan 10 V, it is output intact (step S54).

[0088] The output voltage may be lowered when it is higher than therated voltage by any other voltage, as long as it is controlled not toexceed the rated maximum voltage. Moreover, the output voltage may belowered in any other steps and in any other decrements.

[0089] In Example 4, it is also possible to output a voltage lowereddown to the rated maximum voltage whenever the input voltage exceeds it.

[0090] With this operation control method, it is possible to control thepiston so that it does not move beyond its maximum amplitude and therebyprevent breakage resulting from collision between the piston and thedisplacer.

EXAMPLE 5

[0091] Example 4 deals with an operation control method whereby theoutput voltage is lowered when the input voltage to the microcomputerexceeds the rated voltage or the rated maximum voltage. By contrast,Example 5 deals with a method whereby the output voltage is controlledby detecting the input voltage to the piston and thus the stroke of thepiston instead of detecting a variation in the input voltage. Forexample, after the refrigerator starts being operated, the outputvoltage, which is commensurate with the stroke of the piston, isdetected, and, if the microcomputer 112 detects that this voltage ishigher than a voltage previously set in consideration of the maximumamplitude of the piston, the microcomputer 112 recognizes that voltageas the limit of the output voltage, and inhibits the voltage from beingincreased further.

[0092] In this way, it is possible to control the piston so that it doesnot move beyond its maximum amplitude and thereby prevent breakageresulting from collision between the piston and the displacer.

[0093] Industrial Applicability

[0094] Stirling cycle refrigerators according to the present inventioncan be used as refrigerating devices in refrigerating apparatus such asrefrigerators, showcases, and vending machines.

1. A Stirling cycle refrigerator comprising a piston that is arrangedinside a cylindrical cylinder and that reciprocates along an axis of thecylinder, a driving power source that drives the piston to reciprocate,an electric power source that supplies an input to the driving powersource, and a displacer that reciprocates inside the cylinder with apredetermined phase difference relative to the piston, furthercomprising: position detecting means that is arranged outside a movablerange within which the piston is permitted to reciprocate and controlmeans that reduces the input supplied from the electric power source tothe driving power source when the position detecting means detects thatthe piston has moved out of the movable range.
 2. A Stirling cyclerefrigerator comprising a piston that is arranged inside a cylindricalcylinder and that reciprocates along an axis of the cylinder, apermanent magnet that is fitted to the piston, a driving coil that isarranged around the permanent magnet with a gap secured in between, anelectric power source that supplies an alternating current to thedriving coil, and a displacer that reciprocates inside the cylinder witha predetermined phase difference relative to the piston, furthercomprising: a position detecting coil that is arranged on both sides orone side of the driving coil coaxially therewith outside a movable rangewithin which the permanent magnet is permitted to reciprocate in amanner interlocked with reciprocating movement of the piston and acontroller that varies a voltage of the alternating current supplied tothe driving coil on detecting an electromotive force appearing in theposition detecting coil when the permanent magnet moves out of themovable range.
 3. A method for controlling operation of a Stirling cyclerefrigerator comprising a piston that is arranged inside a cylindricalcylinder, a permanent magnet that is fitted to the piston, a drivingcoil that is arranged around the permanent magnet with a gap secured inbetween, an electric power source that supplies an alternating currentto the driving coil, and a displacer that reciprocates inside thecylinder with a predetermined phase difference relative to the piston,wherein, when the permanent magnet moves out of a movable range withinwhich the permanent magnet is permitted to reciprocate in a mannerinterlocked with reciprocating movement of the piston, and as a resultan electromotive force appears in a position detecting coil that. isarranged on both sides or one side of the driving coil coaxiallytherewith outside the movable range of the permanent magnet, a voltageof the alternating current supplied to the driving coil is varied.
 4. Amethod for controlling operation of a Stirling cycle refrigerator, themethod comprising providing a free-piston-type Stirling cyclerefrigerator having a piston that reciprocates inside a cylinder by useof a gas bearing and a driving power source that drives the piston, andoperating the Stirling cycle refrigerator by applying a voltage to thedriving power source, wherein, when the Stirling cycle refrigeratorstarts being operated, the driving power source starts being operated bybeing fed with a lowest voltage that permits the gas bearing to functionas such, and then the voltage is gradually increased up to apredetermined voltage.
 5. A method for controlling operation of aStirling cycle refrigerator, the method comprising providing afree-piston-type Stirling cycle refrigerator having a piston thatreciprocates inside a cylinder by use of a gas bearing and a drivingpower source that drives the piston, and operating the Stirling cyclerefrigerator by applying a voltage to the driving power source, wherein,when the Stirling cycle refrigerator stops being operated, the voltageapplied to the driving power source is gradually reduced to a lowestvoltage that permits the gas bearing to function as such, and then thevoltage is turned to zero.
 6. A method for controlling operation of aStirling cycle refrigerator, the method comprising providing a Stirlingcycle refrigerator having a chiller that produces cold, a heat rejectorthat produces heat, temperature detecting means fitted individually tothe chiller and the heat rejector, a piston that reciprocates inside acylinder, and a driving power source that drives the piston, andoperating the Stirling cycle refrigerator by applying a voltage to thedriving power source, wherein the temperature detecting means detects atemperature difference between the chiller and the heat rejector of theStirling cycle refrigerator when the Stirling cycle refrigerator is notin operation, and, the greater the temperature difference, the fasterthe voltage applied to the driving power source when the Stirling cyclerefrigerator starts being operated is increased.
 7. A method forcontrolling operation of a Stirling cycle refrigerator, the methodcomprising providing a Stirling cycle refrigerator having a piston thatreciprocates inside a cylinder and a driving power source that drivesthe piston, and operating the Stirling cycle refrigerator by applying avoltage to the driving power source, wherein, when an input voltage ishigher than a predetermined voltage, a voltage